The present invention relates to a process for simultaneously pregelatinizing and drying granular starches and to the pregelatinized starches produced thereby. More particularly it relates to the use of the hot exhaust gases and sonic energy from pulse jet combustion engines to carry out the simultaneous pregelatinizing and drying.
Cold-water-dispersible starches, often referred to as pregelatinized starches, are typically prepared by thermal, chemical, or mechanical gelatinization. The term "gelatinized" or "cooked" starch refers to swollen starch granules which have lost their polarization crosses and which may or may not have lost their granular structure. The thermal processes generally used to prepare such starches include drum-drying, extrusion, and spray-drying. Each of these processes suffer from one or more disadvantages.
Drum-drying involves simultaneously cooking and drying a very high viscosity, semi-solid starch paste on heated drums. The dried sheets are scraped off the drum with a metal knife and then ground. This process can be conveniently carried out at a high starch solids content (typically maximum of about 43%). Despite the fact that it suffers from several product disadvantages and frequent maintenance problems, it is still the most widely used commercial process.
Drum-dried products are subjected to severe mechanical shear on the drum and, as a result, show an initially high viscosity (because of the extensive granular destruction) but then "breakdown" to much lower viscosities with continued shear and/or heating. It is also difficult to maintain high levels of product cleanliness. Some metal contamination during removal of the sheet can occur. Further, the drum system is "open" so that the paste adhering to any part of the equipment (e.g., rolls, dams, scrapers, and/or knife holders) can dry out, darken, and flake into the final product.
Operating costs are very high because of the high torque required to rotate multiple rolls containing the semi-solid starch paste for application to the heated drums. End dams, which are used to prevent leakage of the starch paste from the end of the cylinder, are also high maintenance devices, as are the knives which must be maintained sharp and true (with respect to the mechanical alignment to the drum) to facilitate sheet removal at each drum rotation. Knife failures lead to "double coating" which reduces heat transfer in those areas.
Extrusion may also be used to simultaneously cook and dry starches (see U.S. Pat. No. 3,137,592 issued June 16, 1964 to T. F. Protzman et al.). This method involves the physical working of a starch-water mixture at elevated temperatures and pressures, causing the gelatinization of the starch, followed by expansion after exiting the die for flashing off the water. The temperature and pressure are generated by mechanical shear between the rotating screw (auger) and cylindrical housing (barrel) of the extruder. The extrudate is then ground. It suffers from the disadvantage that one cannot conveniently prepare a high viscosity starch product.
When the moisture of the starch feed is low (i.e., about 20%), a low moisture, expanded, easily ground product is obtained which does not require further drying. However, disruption of the starch granules is so extensive that the product cannot provide the high viscosities needed for most food applications. With high moisture feeds (i.e., 40-50%), the shear forces are significantly reduced and starches with much higher viscosities are produced. However, the extrudate is then moist, soft textured, and must subsequently be dried in a separate drying process.
Pregelatinized starches can also be prepared by spray-drying. In the typical process, an aqueous slurry of the starch is precooked prior to atomization into a large chamber carrying a stream of hot air. The atomization (i.e., breaking the feed into very fine particles) is accomplished with high pressure single-fluid nozzles, with two-fluid nozzles in which compressed air or steam is the atomizing medium, or with a rapidly rotating centrifugal disc.
The use of conventional atomizers presents no problem when an uncooked starch slurry is used. However, when the starch is in a gelatinized (cooked) state, spray-drying the resultant paste becomes more difficult and complex due to the increased viscosity. For example, a dispersion with 7% solids of gelatinized corn starch has a viscosity of about 500 centipoises (cps.), whereas a dispersion with similar amounts of ungelatinized corn starch has a viscosity similar to that of water, i.e., about 1 cps. Not only are gelatinized starch pastes difficult to pump and atomize due to their high viscosities, but the swollen starch granules are subjected to substantial shearing action which destroys the granule's integrity. Hence, native unconverted starches, with or without derivatization, must be processed at low solids. These so-called "thick-cooking starches" must be precooked at about 5-8% solids so that the paste is low enough in viscosity to permit atomization. The use of such a low solids paste is rarely justifiable on an economic basis.
Due to these problems spray-drying is usually limited to "thin-cooking starches", i.e., converted starches where the polymeric structure has been severely degraded by acid hydrolysis, oxidation, and/or dextrinization. Converted starches can be used at higher solids because their pastes are lower in viscosity and can be atomized. However, their viscosity is low only relative to the native unconverted starches. Even a granular acid-converted starch, which shows an extensive viscosity reduction due to degradation, must be processed at less than 30% solids.
Further, a spray-dryer is not an efficient heat transfer device with respect to providing the energy needed for evaporation of water. The equipment tends to be massive in size, e.g., 4.6-7.3 m. (15-24 ft.) in diameter by 6.1-12.2 m. (20-40 ft.) in height. Convective and radiant heat losses and leakage of ambient air are exaggerated by the large surface areas. Also, large volumes of discharged air (e.g., 566-1133 m..sup.3 or 20,000-40,000 ft..sup.3 per min.) at exit temperatures of 82.degree.-107.degree. C. (180.degree.-225.degree. F.) carry a large portion of input energy. The heat efficiency is probably only about 50-55%.
The improved spray-drying method disclosed in U.S. Pat. No. 4,280,851 (issued July 28, 1982 to E. Pitchon et al.) overcomes the solids/viscosity limitation of the precooked starch feed by introducing an aqueous slurry of a granular starch to a set of atomizing orifices. The atomized slurry then contacts high pressure steam in an outer chamber of the same nozzle which has dual atomizing chambers. The steam serves to cook the starch, apparently while the starch is in an atomized state, and provides energy for atomizing the water/starch/ steam mixture through the orifice of the outer chamber. Notwithstanding the advantages of using an uncooked starch slurry, this process suffers from many of the disadvantages of conventional spray-drying. The proportion of heating medium (e.g., steam) to liquified material (an aqueous slurry at about 40% starch solids) is about 0.5-3.0:1. Test experience indicates a ratio of at least 1.25-1.50:1 is needed for good gelatinization to a high viscosity product. At a ratio of 1.5, a 40% solids feed is diluted to about 16% solids which approaches the feed solids of a precooked starch paste. Steam consumption would probably be high (perhaps 100 hp/1000 lb. starch/hr.), and the dryer hot air stream would have to be at a relatively high rate to maintain an acceptable humidity. Thus, the inherent low efficiency of spray-drying, described above, is still working to negate many of the advantages of this process.
Hence, there is still a need for an efficient and versatile process for preparing cold water dispersible (i.e., pregelatinized) starches.